Several techniques are known in the art for growing crystals. The Czochralski (CZ) process is the most widely used technique for growing crystal ingots used in the manufacture of integrated circuit (IC) chips. In the CZ process, a crystal puller system grows solid, single crystal ingots from melted charge material. High-quality ingots are substantially free of defects, have a uniformity of characteristics throughout the ingot, and are largely uniform from one ingot to the next.
A modern CZ crystal puller typically includes a crucible which holds a liquid melt form of a charge material, such as silicon from which the crystal is to be grown. The crucible is surrounded by a furnace that heats the charge to a melted state (the "melt"). The crucible and furnace are located in a sealed vacuum chamber which includes the components: a growth chamber that surrounds the furnace, a transition chamber located above the growth chamber and an elongated receiving chamber which holds the crystal as it grows. The vacuum chamber allows the environment around the growing crystal to be controlled. A vacuum slide valve located between the receiving chamber and the transition chamber can be used to isolate the two chambers. The vacuum chamber components typically are bolted together with vacuum seals between the pieces.
During the pulling process, a pull head mounted on the top of the receiving chamber holds a "seed" crystal at the end of a vertical rod or a cable and lowers the seed to contact the melt. Upon contacting the melt, the seed causes a local decrease in melt temperature, which causes a portion of the melt to crystallize around the seed. The seed is then slowly withdrawn from the melt, passed through the transition chamber and passed into the receiving chamber by the pull head. As the seed is withdrawn, the portion of the newly-formed crystal that remains within the melt essentially acts as an extension of the seed and causes more melt to crystallize around the seed. This above process is continued until the crystal is grown to the desired size, e.g., eight to ten feet long and weighing about 150 Kg. Eventually, the finished crystal is lifted entirely into the receiving chamber, which is then detached from the remaining portion of the vacuum chamber system so that the crystal may be retrieved.
To lift the seed and eventually the ingot, a typical pull head includes a controllable winch having a cable affixed to the seed and the attached ingot. Conventionally, the pull head is attached directly to the top of the receiving chamber, so that, as the receiving chamber is moved or detached, the pull head moves with it. To improve an ingot's uniformity, in many systems, the pull head is mounted on a plate that is rotatable with respect to a base which is, in turn, is mounted on the receiving chamber. The rotatable plate allows the pull head to rotate around a vertical longitudinal axis of the crystal during the pulling operation so that the pulling cable, the seed crystal and the ingot rotate around the crystal longitudinal axis as the ingot is withdrawn from the melt. In addition, to further improve uniformity, the crucible containing the melt is often rotated about a vertical axis parallel to the same crystal longitudinal axis as well.
To insure uniform crystal growth, the pull head must be carefully aligned relative to the crucible so that the axis of rotation of the cable system aligns with the axis of rotation of the crucible. Further, the pull head itself must be positioned so that its rotational axis is vertical. However, the position of the receiving chamber relative to the rotational axis of the crucible is dependent on the tolerance stack-up of the base chamber, grow chamber, transition, isolation valve and receiving chamber. This structure renders it difficult to achieve an accurate alignment. Furthermore, the receiving chamber must be removed from the vacuum chamber structure and then replaced during each crystal pulling process. Each time the receiving chamber is replaced, it may not be returned to its exact previous position due to the tolerances in the chamber pieces. Consequently, it is difficult to exactly align the pulling cable to the pulling head axis and the crucible rotational axis. Further, the pulling head may become tilted so that the rotational axis of the pulling head varies from true vertical.
It has been found that this misalignment among the pulling cable, the pulling head axis, and the crucible rotational axis, as well a tilting of the pulling head, will deteriorate the growing condition of the crystal and render the rotating pulling cable/crystal susceptible to an orbiting motion which may be very harmful to the crystal quality and may even disrupt the crystal growing process.
Consequently, there is a need in the art for improved methods and apparatus for growing crystal ingots that provide stable, repeatable growth of crystal ingots.
It is an object of the invention to provide an improved method of, and apparatus for, pulling a crystal ingot from a melt.
It is another object to increase the stability of the crystal growing process, primarily by improving the ability to align the pulling cable and improving the repeatability of the alignment.